Dynamic light scattering (DLS) is a powerful technique for measuring the size of particles, for example nanomaterials and nanoparticulates. Typically, a suspension or solution containing the particles is placed in a transparent sample cuvette and is illuminated with a vertically polarised laser beam. Light scattered from the particles along a particular direction is detected in a plane orthogonal to the polarisation of the incoming light, and analysed using well known methods to determine properties of the particles. For example, light that is back-scattered from the particles may be detected, and/or light that is forward scattered and/or side-scattered (e.g. scattered light at an angle of 90° to the incoming light).
A further development of dynamic light scattering, depolarised dynamic light scattering (DDLS), seeks to quantify the anisotropy in scattering from particles that deviate from a sphere of uniform refractive index, such as rods, ellipsoids, discs, or spherical Janus particles. In DDLS, a particular polarisation of scattered light is detected. For example, a sample may be illuminated with vertically polarised light, and the correlation function of the scattered light detected in each scattered polarisation, separately, along the detection path.
There are many laboratory instruments available that can perform dynamic light scattering, such as the Malvern Zetasizer® range. However, these instruments may not be able to measure different polarisation states of the scattered light, and so cannot perform depolarised dynamic light scattering measurements. Upgrading system hardware, or replacing a system altogether in order to perform DDLS may be undesirably expensive.
Further information about the particles in a solution can be found from fluorescence correlation spectroscopy (FCS), which uses a correlation analysis of fluctuations in florescence emitted by the particles to determine particle dynamics. The equipment needed to perform FCS is similar to that for DLS, but requires an appropriate set of optical filters to be used to analyse the scattered light.